9.1: Saccharomyces cerevisiae

Status

reviewed by CK-12

Common Names

Yeast

Baker's Yeast

Brewer's Yeast

Budding Yeast

Description

Saccharomyces cerevisiae, which in Latin means “sugar fungus,” is a species of the fungi yeast. It is a single-celled eukaryotic organism. S. cerevisiae has been instrumental in baking and brewing for thousands of years. S. cerevisiae reproduces by budding. Often when using the general term yeast, scientists are referring to S. cerevisiae.

Saccharomyces is perhaps the most useful and one of the best studied single-celled eukaryotes. As such, S. cerevisiae is considered a eukaryotic model organism in molecular and cell biology, and has been instrumental in research on the cell cycle, metabolism, signal transduction, protein interactions, and regulatory networks.

Domain: Eukarya

Kingdom: Fungi

Phylum: Ascomycetes

Class: Saccharomycetes or Hemiascomycetes

Order: Saccharomycetales

Family: Saccharomycetaceae

Genus: Saccharomyces

Species: S. cerevisiae

Habitat

S. cerevisiae's natural habitat is on the surface of fruit, but it is best known for its role in the baking and brewing industries.

Biology

Cell Biology

S. cerevisiae can exist as both haploid and diploid cells. The haploid cells undergo a simple life cycle of growth followed by mitosis. Under stressful conditions, the cells will die. The diploid cells also undergo growth and mitosis, although under stressful conditions, they will undergo meiosis and produce haploid spores. Under favorable conditions, the spores will then mate, producing a diploid cell.

Saccharomyces are heterotrophs, obtaining their energy from glucose, utilizing both aerobic and anaerobic respiration.

S. cerevisiae is a model organism in molecular and cell biology studies. Many signalling pathways of importance in many eukaryotic organisms were first discovered in this species. Many common cell and molecular characteristics of eukaryotic cells were also identified in yeast.

Yeast two-hybrid screening is a molecular biology technique used to discover protein–protein interactions and protein–DNA interactions by testing for physical interactions between two proteins or binding between a single protein and DNA. This technique was developed in 1989, and was originally designed to detect protein–protein interactions using the GAL4 transcriptional activator of S. cerevisiae. GAL4 is a nuclear protein, acting as a positive regulator of gene expression for galactose-induced genes. The basic idea behind the test is the activation of downstream reporter gene(s) by the binding of a transcription factor onto an upstream activating sequence (UAS) on the DNA.

Genetics

The genome of S. cerevisiae was the first eukaryotic genome that was completely sequenced. The sequence was released to the public on April 24, 1996. The genome is composed of about 12,156,677 base pairs and 6,275 genes, distributed on 16 chromosomes. Only about 5,800 of these genes are believed to be functional. This species is estimated to share about 23% of its genome with that of humans.

One of the first groups of growth mutants isolated from yeast were those that required adenine, one of the bases in DNA and RNA nucleotides and a component of ATP. There are several genes in yeast that are involved in adenine synthesis, and a mutation in any one of these genes requires that adenine be added to the growth medium. The first two adenine-requiring mutants discovered were in yeast, and are known as ade1 and ade2. The functional alleles of these genes are the ADE1 and ADE2 genes.

Evolution

Recent studies estimate the arrival of fungal organisms at about 760–1060 million years ago (Ma) on the basis of comparisons of the rate of evolution in closely related groups. For much of the Paleozoic Era (542–251 Ma), fungi lived in water. Moving onto land required fungi to develop other methods to obtain nutrients, including parasitism, saprobism, and the development of mutualistic relationships, such as mycorrhiza and lichen. Recent studies suggest that the ancestral ecological state of the Ascomycota was saprobism.

The fungi probably colonized the land during the Cambrian (542–488.3 Ma), long before land plants. Fungal fossils do not become common until the early Devonian (416–359.2 Ma). At about this same time, approximately 400 Ma, the Ascomycota and Basidiomycota diverged. Yeast is a member of the phylum Ascomycota.

Ecology

Physiology

S. cerevisiae can exist as both haploid and diploid cells. Haploid S. cerevisiae cells exist as two mating types: a and α. Both haploid and diploid S. cerevisiae cells may reproduce asexually by budding. But yeasts, like all fungi, may have also have sexual reproductive cycles. Under stressful conditions, haploid cells will generally die; but under the same conditions, diploid cells can enter sexual reproduction processes. Under stressful conditions, diploid yeast cells, can undergo meiosis to produce four haploid spores: two a spores and two α spores. Upon favorable conditions, cells of opposite mating type can fuse to form a diploid cell. There are two alleles that determine the mating types of haploid strains. Alternative alleles of the MAT gene, MATa and MATα, determine the two opposite mating types.

Yeast have simple nutritional requirements. As yeast are fungi, they are unable to carry out photosynthesis. They obtain their carbon from carbon containing compounds, which can be as simple as the two-carbon acetate. In addition, they also require a source of nitrogen, which can be as simple as the single-nitrogen containing ammonium sulfate. Yeasts can also use a variety of organic nitrogen compounds containing both carbon and nitrogen, such as urea and various amino acids. The only other complex compound that yeast require is the vitamin, biotin. In addition, they also require more simple molecules including a variety of salts and trace elements.